When you place an item in service, you use the purchase price or fair market value, whichever is lower at the time. There are various ways to take the depreciation faster than over the full lifetime of the asset, including the safe harbor for items under $2500, bonus depreciation, and section 179 depreciation.
I believe that, to take the $2500 de minimis safe harbor election, you must have actually paid for the item during the year you want to claim the expense. See here,
https://www.irs.gov/publications/p535#en_US_2021_publink100075803
This safe harbor would not apply to your computer if purchased for personal use in 2020 and converted to business use in 2022.
I believe you can take section 179 depreciation for used property when placed in service for the first time in business. This will have the same effect as allowing you to deduct the entire FMV the first year, but you will have to deal with deprecation recapture if you take the computer out of service before the end of its useful life.
Even if you did expense the computer, you may have to recapture depreciation if you sell it later (even as used), since your adjusted cost basis after claiming the safe harbor would be reduced by the amount you claim as FMV.
If you list the computer as an asset in Turbotax, the program should hopefully advise you on what options you have.
